The invention relates to switching power converters. More particularly, the invention relates to a technique for limiting current by limiting a switching duty cycle in such a power converter.
In a conventional switching power converter, provision may be made for limiting current to a load in the event the load experiences a fault. For example, U.S. Pat. No. 5,742,151 discloses a PFC-PWM power converter in which the output current developed by the PWM section is limited in when an excessive output current is detected. More particularly, for each switch cycle, the switch is closed upon sensing an excessive output current. A drawback to this technique is that when the switch is opened for each switching cycle, an excessive current can occur before the switch is closed in response to the excessive current. This repeated occurrence of excessive current may cause excessive power dissipation in the converter.
What is needed is an improved current limiting technique for a switching power converter. It is to these ends that the present invention is directed.
The present invention is an improved technique for limiting current in a switching power converter. The power converter includes a switch for transferring energy from a source to the load by opening and closing the switch according to a duty cycle. A duty cycle limiter is triggered when the output current of the converter exceeds a predetermined level. The duty cycle is limited to an amount that is related to a degree to which the output current is excessive. In one aspect, the duty cycle limiter include a capacitor having a charge that is gradually changed in response to the output current exceeding the predetermined level where the duty cycle is related to a voltage on the capacitor. In another aspect, the duty cycle limiter includes a counter where the duty cycle is related to a count of the counter. In another aspect, the duty cycle limiter provides a soft-start in which the switching duty cycle is gradually increased upon powerup. Once the converter is operating normally, the duty cycle is controlled to regulate the output voltage.
The invention has an advantage in that by reducing the switching duty cycle, the output current is reduced and is, thus, less likely to reach an excessive level upon each cycle of the switch. In comparisons to prior techniques, this reduces power dissipation in the converter.